1. Field of the Invention
The present invention relates to a semiconductor device comprising a plurality of semiconductor chips stacked in the direction of thickness, and a method for making the same.
2. Background Art
There is known a type of semiconductor device in which a plurality of semiconductor chips are stacked and sealed in a resin package for drastically increased circuit mounting density. Such a semiconductor device is called “chip-on-chip” type because of the stacking arrangement in which one of the chips is mounted on another.
FIG. 29 shows an arrangement of a prior art chip-on-chip semiconductor device. As understood from the figure, in this arrangement, a plurality of semiconductor chips 9a-9c are stacked successively on a surface of a substrate 90. According to such an arrangement, an area occupied by the semiconductor chips 9a-9c on the surface of substrate 90 is small, advantageously increasing the mounting density of the semiconductor devices.
However, the prior art has following problems.
Specifically, when the plurality of semiconductor devices 9a-9c are stacked, respective electrodes 94a-94c are elevated accordingly to higher locations. As a result, the electrodes 94c of the highest semiconductor 9c are located disadvantageously high (height difference Ha), away from the surface of substrate 90 formed with terminals 92 to which the electrodes 94a-94c are to be connected via respective pieces of wire 93.
Under such a situation, it is sometimes difficult to make a proper connection between the electrode 94c and the terminal 92 because of the big height difference Ha. Specifically, in a conventional wirebonding machine, a capillary can perform a proper bonding only within a vertical range of ±300 μm away from its baseline height. Sometimes, however, the height difference Ha is greater than the upper limit (i.e. greater than the baseline height added with +300 μm), making impossible to use the wirebonding machine for the wirebonding operation. In addition, when the height difference Ha is large, as shown in FIG. 30, the capillary 95 has to be significantly tilted when the capillary 95 presses the wire 93 to the electrode 94c or the terminal 92. Under such a circumstance, there develops a space S between a tip portion of the capillary 95 and the electrode or the terminal, making impossible to properly bond the wire 93. As a result, according to the prior art, it is difficult to properly bond an end of the wire 93 to the electrodes 94a-94c or the terminal 92, and there is a significant risk of causing a faulty connection at the location where the wire is bonded.